This invention relates to color cathode ray tubes of the type having a shadow mask, and especially to a system for mounting a shadow mask on the faceplate of a color tube. This invention has applicability to mounting systems for shadow masks of various types, including post deflection focus masks.
Conventional color cathode ray tubes have a shadow mask assembly which includes a massive frame to which is welded a dished, apertured mask. The frame is, by design, extremely rigid and provides rigidity for the mask. The mask-frame assembly is mounted in a conventional tube by a suspension system comprising three or four leaf springs which are welded to the frame at spaced points around the periphery thereof. These springs must be relatively stiff to support the heavy mask-frame assembly, typically applying a load of 4-5 pounds or more to the mask-frame assembly. The springs have apertures at their distal ends which engage studs projecting inwardly from a rearward flange on the tube faceplate. The mask-frame assembly is capable of being demounted and precisely remounted in a tube by depressing the springs to disengage the said studs. This type of system has proven to be commercially viable, however, the mask-frame assembly and its suspension system are undesirably expensive.
The invention is preferably (though not necessarily) employed in a color tube having as components a unique bulb with a flangeless faceplate and a unique corner-mounted shadow mask. In this tube, a low cost, lightweight, non-self-rigid, torsionally flexible mask is provided. The flangeless faceplate imparts the necessary rigidity to the mask. A four corner mask mounting system provides a mechanically rigid link between the faceplate and the mask and yet permits the mask to be conveniently and repeatably demounted and precisely remounted in the tube, as is required by present tube manufacturing practices.
The advantages of this tube are manifold. A primary advantage resides in the appreciable savings in bulb cost, and from the use of a lightweight, low cost shadow mask which is preferably of one-piece, frameless construction. Early versions of such a system are shown in the U.S. Pat. No. 3,912,963 and in U.S. Pat. No. 3,943,399.
A mask and mask suspension system of the nature described has imposed upon it a number of requirements and constraints not presented in conventional systems in which a rigid frame is used to impart rigidity to the mask. Before enumerating these requirements and constraints, a discussion of certain principles underlying this invention will be engaged. A shadow mask of the type with which this invention is concerned may be modeled as a rectangular four bar linkage affixed to a flexible sheet. Such a model is shown in FIG. 18A. The four rigid bars of the linkage are designated A, B, C and D; the sheet is labeled S. As is well known, a four bar linkage is not inherently a rigid structure. The rectangular four bar linkage, in its free state, might, e.g., quite easily be skewed into a parallelogram geometry. It is evident, however, that the FIG. 18A model cannot be skewed in its plane to take a parallelogram shape since it is affixed to the sheet S.
The linkage can, however, be torsionally twisted about its diagonals, as shown for example in FIG. 18B. In FIG. 18B, the model has been twisted as follows -- the linkage bar A has been rotated toward the reader (see arrows); the linkage bar C has been rotated away from the reader. The corners 1 and 3 have been displaced upwardly and the corners 2 and 4 have been displaced downwardly. The sheet S is thus stressed convexly along diagonal 2-4 and somewhat concavely at the ends of diagonal 1-3. The model may thus be thought of as being twisted about one of its diagonals (here shown as diagonal 1-3). It can be noted that the model configuration, after twisting, is changed substantially less along its major axis M.sub.a and minor axis M.sub.i, than along the diagonals. Thus a four bar linkage affixed to a flexible sheet is relatively stiff with respect to its major and minor axes (due to the rigidity of the bars), but is relatively flexible in torsion. When torsionally flexed (twisted), about its diagonals, the corners are displaced, but points on the major and minor axes remain relatively stationary.
As will be pointed out in more detail hereinafter, the shadow mask with which this invention is concerned is similar to the described model in its mechanical characteristics.
As suggested, the principles of this invention, though not limited to such application, are most useful when embodied in a color cathode ray tube having a flangeless faceplate. When such a faceplate is sealed to mating funnel after completion of the faceplate screening and mask insertion operation, the faceplate is very apt to experience a twist-wise elastic distortion due to a tolerance-related configurational mismatch between the funnel and faceplate sealing surfaces. Any such distortion will be rendered a permanent deformation when the sealing cement has cured and the sealing operation is completed. Thus, one of the necessary general requirements imposed on a mask and mask-suspension system intended for use with a flangeless faceplate is that it must be able to adapt to such twist-wise deformations of a faceplate with which it is mated. Stated another way, the mask must be capable of flexing or twisting about its diagonals in much the same way faceplates are apt to twist-wise deform in their contour during tube fabrication, and its suspension system must provide for such adaption. As will become evident as this description proceeds, the shadow mask and four-corner suspension system with which this invention is concerned are uniquely capable of meeting this requirement.
An important requirement imposed on such a mask suspension system is that, since the mask is non-self-rigid, the suspension system for the mask must effectively transfer the rigidity of the faceplate to the mask. That is, the suspension system must be very rigid in the tube's axial direction. Axial rigidity is also important in establishing and maintaining a prescribed position of the mask in space relative to other tube components.
Another important requirement imposed on such a suspension system is that it precisely fix and hold a predetermined spatial position of the mask as a whole relative to the faceplate against translational or rotational displacement, in spite of any thermal expansion or contraction of the mask, demounting and remounting of the mask, or mechanical shocks. It has been found that this requirement translates into a requirement that the suspension system be very stiff in the angular directions, that is, in the directions perpendicular to the faceplate diagonals and to the central axis of the tube. If the mask suspension system is not sufficiently stiff in the angular directions, the mask is apt to not always return to its bogey position (nominal assigned position) after having received a mechanical shock, or having been demounted and remounted, or after a thermal cycle. This fact is due largely to the mass of the mask, to friction at the points of engagement of the mask-mounted and envelope-mounted components of the mask suspension devices, and to the specific movements occurring with each individual insertion.
It is also desirable that any thermally induced movement of any part of the mask or of any mask suspension element during tube operation be radial, rather than angular, since radial errors can be compensated by making adjustments in the beam deflection characteristic, whereas angular errors cannot be.
It is a related and very important requirement that the mask suspension system impose as low as possible (ideally zero) radial loading on the mask. Ideally, the radial loading of the mask by its suspension system during thermal cycling of a tube-in-process and during ultimate tube operation is very low. The reason for this desired low radial loading involves the mechanical construction of the mask. The unique mask with which this invention is associated does not have the conventional heavy, rigid supporting frame, but rather is deliberately caused to be of low mass and somewhat flexible, particularly in torsion. It has been found that a too-high radial loading of the mask is apt to cause radial compression of such a mask, resulting in radially inward registration shifts during thermal processing of the tube and/or during tube operation. This problem has been found especially severe if the mask is manufactured by a method wherein it is preformed before it is etched. The etching process causes the mask to lose a substantial part of its stiffness. If a high radial loading is imposed on such a mask, the mask is apt to take a different contour after etching than it had before etching.
A major motivation behind the development of the unique tube described is to reduce the cost of tube manufacture. It is therefore an extremely significant object that the mask suspension system be as inexpensive as possible.
The referent copending applications and patents depict a number of highly successful approaches to meeting the afore-described requirements and constraints on mask suspension systems of the nature described. The referent copending application Ser. No. 675,653 describes a system in which cost reduction in the mask suspension system is effected by forming integral, radially oriented modifications of the faceplate glass in the faceplate corners which make six point engagement with mounting elements on the mask corners. The present invention utilizes the teaching of that application (of providing an integral radial modification of the faceplate) in the interest of cost reduction, but is believed to be an improvement over the system described in that application by reason of a construction which makes possible further economies, and which has other advantageous properties, which will become evident from the ensuing description of this invention.
A second approach is described and claimed in the referent U.S. Pat. No. 3,986,072. According to the teaching in that patent, a mask suspension system is provided which utilizes a stud affixed to the tube envelope in each corner thereof which engages a mask-mounted component on the corner of the mask. Either the envelope-mounted component or the mask-mounted component employs a cantilevered leaf spring which is extremely stiff in the angular and axial directions. According to the teaching of the said patent, the spring applies a relatively low load on the mask; however, it has been found that for a variety of reasons, it is not without some effort that the relatively low loading of the mask is established and maintained.
It is another object of this invention to provide a system lower in manufacturing cost than the system described in the said U.S Pat. No. 3,986,072.
As is evident from the above, I do not claim to be the first to conceive of the idea of suspending a lightweight, flexible shadow mask for a color CRT by means of integral modifications of the inner surface of the supporting faceplate which make six point contact with elements spaced around the mask. As noted above, a system along these general lines is described in the referent application Ser. No. 675,653. Also, U.S. Pat. Nos. 2,961,560 -- Fyler, 3,038,096 -- Knochel et al, 3,824,989 -- Christofferson, 2,932,241 -- Haas, and 2,824,990 -- Haas, each show color cathode ray tubes having a mask suspension system in which a lightweight, flexible mask is supported on the inner surface of a color CRT faceplate by integral glass protuberances which project from the faceplate and make positional engagement with V-channels formed in or attached to the shadow mask. None of these references discloses a four-corner suspension system; rather, each discloses an arrangement of supporting the shadow mask relative to the faceplate at the conventional locations around the periphery of the faceplate. However, as apparently recognized by certain of those patentees, such a support system, when used with a flexible mask, is unsatisfactory unstable. Accordingly, other points of mask supports are provided, apparently to stabilize an otherwise unstable mask.
In the prior U.S. Pat. No. 2,961,560 to Flyer, engagement of the V-channel structure on the mask with the integral protuberance on the faceplate is intended to permit a frictional sliding engagement of the V-channel over the protuberance as the mask expands and contracts during heating and cooling. As taught by Fyler (see col. 3, lines 17 et seq. and col. 5), if such provision is not made, thermal expansion and contraction of the mask will deform the mask and result in loss of beam-phosphor registration. The patents to Knochel et al and the Haas patents disclose systems which are similar to that of Fyler in the sense that V-channels on the mask are supported by rounded protuberances on the faceplate -- yet no mention is made of compensation for such thermal expansion and contraction of the mask. The inevitable conclusion is that, since the patents are presumed to describe operative structures, the Haas, Christofferson and Knochel et al systems must also provide for relative movement of the V-channels and supporting posts as the masks in their systems expand and contract. From my working experience, however, it is clear that such an arrangement would be inoperative since the sliding frictional engagement between mask and faceplate would not result in precise positioning of the mask relative to the faceplate. As a mask such as taught by the Fyler, Knochel et al or Haas patents would expand, the contact point between the V-channel and the mating glass protuberance would change. Due to the inevitable manufacturing tolerances in mask and glass manufacture, as the contact point would change, the position of the mask relative to the faceplate would shift, resulting in beam-phosphor registration errors.
In addition, it is believed that any system which requires six-point hold-down of the mask (Fyler), or ten-point hold-down as in Knochel et al, or five-point hold-down as apparently shown by Haas, is an unworkable structure due to the distortion of the mask required to make engagement with the large number of support points around the periphery of the faceplate. Also, each of the described patent systems is believed to be commercially unacceptable for cost reasons alone.
______________________________________ Other Prior Art U.S. British ______________________________________ 2,222,197 3,484,638 1,278,633 2,733,366 3,497,746 1,278,634 2,823,328 3,521,104 1,278,635 2,906,904 3,529,198 1,172,334 2,916,644 3,537,159 2,922,063 3,548,235 3,350,593 3,639,799 3,404,302 3,700,949 3,450,920 3,735,179 ______________________________________